thru hull exhaust conversion

[lahnlubba]

i have a 1973 sea ray with an omc 225 hp,.. we are going to change the motor and replace it with a reverse cooled lt1 motor originally from a 1993 corvette. We want to put the exhaust out thru the hull, but keep it about 3 ins below the water line for quiet idling,.. has any one else done this? has anyone any advice as to what to be careful of etc,..sites to go to to read necessary info? any positive input would be greatly appreciated,..thanks to all<br />nick

 

[Ralph 123]

Re: thru hull exhaust conversion

Nick,<br /><br />You need to pick up a copy of "Chevy Marine Performance" The book goes into great detail about the difference between an auto engine and a marine engine. the engine needs to be converted for marine use - if you don't kaboom! And I don't mean just a blown engine! You may already know all this but just in case I though I'd mention it.<br /><br />With the exhaust that low, how are you going to keep a waves from hitting your stern and sending water right down into the engine? 3" is nothing...

 

[McKenzie]

Re: thru hull exhaust conversion

If your going to put the exhaust 3" below the waterline, you might as well stay with the prop exhaust. If you really wanna do it that way, make sure you get exhaust tips with flapppers, and make the exhaust routing have steep drops to keep water from backing into the engine. You may want to consider riser spacers. I imagine that engine has a roller cam, so you need a foot drop MINIMUM (riser to exhaust outlet)to keep it from sucking water into itself. Running a setup like yours may require even more. If I were you, I'd either go with the prop exhaust, or go 3" above the waterline.

 

[McKenzie]

Re: thru hull exhaust conversion

You may or may not know this, but for an auto engine to be used in a marine application, you need a marine alternator, starter, carb/efi, distributor, and waterpump. Are you going to swap all the stuff from the old engine? Are you going to use a closed cooling system? If so, you could get away with an automotive waterpump.

 

[denniz]

Re: thru hull exhaust conversion

Check out Corsa performance for the thruhulls and accessories.

 

[lahnlubba]

Re: thru hull exhaust conversion

ralph,.. thanks, could be good reading,i've built a few motors over the years, have a basic idea 'tween auto and marine,.. <br />Mckenzie, the starter and alternator and carb will switch over ok,..the motor right now uses an automotive waterpump,feeds in thru the leg,<br />(i just did the impeller and ball gears, and re-sealed the leg..whew! ) it has open cooling, freshwater for me up here,.<br />I measured from the end of the manifolds to the water line and there is an 11" drop no risers, log style manifolds,)looking at the corsa site the pipes would come down the same at a 45-60deg angle,.. I think tho that 3" below would be too deep, altho the boat "rises" when she is moving quick,.. i think with a 45 deg cut and thru the hull by about 2" and the upper edge only 1" below the surface will work better, With flappers of course,..<br />The motor has a close lobe centre cam in it (114deg)not much overlap just plenty of lift,.It'll idle nicely at 500 -600 rpm with no "lope" makes about 70ft lbs of torque more than the motor it will replace, <br />Rod knocker, thanks for a great site,.. kinda expensive and shiny tho, for a 'POH" cdn boy with an old ray,.. <br />thanks again <br />nick.

 

[McKenzie]

Re: thru hull exhaust conversion

I know i sound like a broken record with the auto/marine differences, but you never know who and who hasn't heard it. I'm still not convinced by this underwater exhaust. I remember in highschool my chem teacher boiled water in a open container, sealed it and let it cool. It collapsed as it cooled off because of the vacuum inside. Now think of the exhaust on your boat. After a hard run and you come to a stop and shut the engine off, the water is going to flow into your exhaust and make a seal at that end of the exhaust pipe at the waterline. The air above the waterline in that pipe is going to be hot (obviously) as it cools, it will create a vacuum, thus drawing the water upward. If you had a really hard run or if you ever overheat, that vacuum will be even stronger, possibly strong enough to raise the water up to the riser. Then the water floods down onto the valves and into the open ones. When you goto start up again, *click*! Hydro-lock. Sorry for the science lesson! Wow, a practical use for science class ? What has me more amazed is i paid attention that day!? Anyway....If you really have your heart set on doing this, find a Volvo penta guy, I think they might make some kind of vacuum break that you install into the risers. definetly look into those! (if they exist )

 

[lahnlubba]

Re: thru hull exhaust conversion

hmmmmm, three inch exhaust, approx 29inches long,.. 11 inch rise,(water is heavy) watercooled manifolds,...<br />i have my doubts,.. specially with flappers on the end,.. <br />nick

 

[flashback]

Re: thru hull exhaust conversion

ya know i thought about this for a while, and decided that although I love the sound of a v8 engine with thru exhaust, I boat on an inland lake of finite size, very crowded, and I started thinking if everyone put thru hull exhaust on their boat the noise would be awful.....So consider where you boat at before you do this, tho there is an option that allows you to use either exhaust called "Captains Choice"

 

[McKenzie]

Re: thru hull exhaust conversion

I have my doubts too if it would happen, but why chance it? This sort of thing does happen to people, so this isnt some theoretical problem. I haven't seen a boat yet that has underwater exhaust like yours, theres probably a reason for that.

 

[akriverrat]

Re: thru hull exhaust conversion

i'm running 2 1/2" pipes that are immersed about 1" below the at rest waterline. wake running up tailpipes on sudden stops was minor problem befor i added flappers. friends zz4 exit 3"pipes about 3" below static waterline. after about 1500 rpm at dock they might as well be above water. we both have center rise manifolds and about 16-18" from static line to top of elbows without any problems. other friends boat exits 3" pipes above water but runs through marine mufflers and is downright civilized.

 

[anesthes]

Re: thru hull exhaust conversion

LT1 intake bolt pattern is not the same as SBC, what are you using for an intake/carb?<br /><br />Also, lt1 uses a crank trigger. You need to use a distributor. <br /><br />I like the LT1 EFI intake design, not sure if I like the block/head design just yet.. <br /><br />-- Joe

 

[lahnlubba]

Re: thru hull exhaust conversion

flash, i "hear ya" captains call..1200 sheesh!<br />anesthes, there is an intake manifold available for this swap,.. takes a regular small block distributor, just gotta plug the opti spark hole in front cover, i run a 95 and a96 impala ss and the beauty of these reverse cooled motors is high compression,nice small combustion chamber,. cools the heads first, these motors have been proven to be exceptionally good performers and excellent longevity, efi would be nice but a bit expensive for me,..<br />river rat,.. thats what i'm talking about, civilised for docking, and performance when 'on the pipe!"

 

[anesthes]

Re: thru hull exhaust conversion

I understand that but I'm not a big fan of the LT1.. 300-320hp on a 11:1 c/r motor is NOT impressive to me in the least. I've built 9.5:1 motors that pushed 450hp.. But a lot of guys seem to be happy with the LT1 stuff.. I agree with the cooling benefit, over the SBC design.<br /><br />-- Joe

 

[lahnlubba]

Re: thru hull exhaust conversion

joe, its a small block chevy,.. so if you built 1 with 450 @9.5, at 11.1 you'll just make more, with no detonation due to the cool heads,..<br />my impala make 345rwhp with just a mild cam and headers,.